sedimentary rock composed mainly of calcium carbonate (CaCO₃), usually in the form of calcite or aragonite. It may contain considerable amounts of magnesium carbonate (dolomite) as well; minor constituents also commonly present include clay, iron carbonate, feldspar, pyrite, and quartz.

Most limestones have a granular texture. Their constituent grains range in size from 0.001 mm (0.00004 inch) to visible particles. In many cases, the grains are microscopic fragments of fossil animal shells.

Limestone has two origins: (1) biogenic precipitation from seawater, the primary agents being lime-secreting organisms and foraminifera; and (2) mechanical transport and deposition of preexisting limestones, forming clastic deposits. Travertine, tufa, caliche, chalk, sparite, and micrite are all varieties of limestone.

Limestone has long fascinated earth scientists because of its rich fossil content. Much knowledge of the Earth's chronology and development has been derived from the study of fossils embedded in limestone and other carbonate rocks. Limestone also has considerable commercial importance. Limestones enriched in phosphate by the chemical action of ocean waters constitute a principal source of raw materials for the fertilizer industry. When heated to temperatures of 900° to 1,000° C (1,650° to 1,800° F), limestones will dissociate calcium carbonate and yield carbon dioxide and lime, the latter having major applications in the manufacture of glass and in agriculture. Certain varieties of limestone also serve as a building stone; they are widely used for flooring, exterior and interior facings, and monuments.